The use of molecular sieves as catalysts in aromatic conversion processes are well known in the chemical processing and refining industry. Aromatic conversion reactions of considerable commercial importance include the alkylation of aromatic compounds such as in the production of ethyltoluene, xylene, ethylbenzene, cumene, or higher alkyl aromatics and in disproportionation reactions such as toluene disproportionation, xylene isomerization, or the transalkylation of polyalkylbenzenes to monoalkylbenzenes. Often the feedstock to such an aromatic conversion process will include an aromatic component, i.e. alkylation substrate, such as benzene, and a C2 to C20 olefin alkylating agent or a polyalkyl aromatic hydrocarbon transalkylating agent. As used herein, terms such as “C4”, “C5”, “C6”, etc. designate the number of carbon atoms per molecule of a hydrocarbon or hydrocarbon specie. In the alkylation zone, the aromatic feed stream and the olefinic feed stream may be reacted over an alkylation catalyst to produce alkylated aromatics, e.g. cumene or ethylbenzene. A portion or all of the alkylation substrate may be provided by other process units including the separation section of a styrene process unit. Polyalkylated benzenes are separated from monoalkylated benzene product and recycled to a transalkylation zone and contacted with benzene over a transalkylation catalyst to yield monoalkylated benzenes and benzene.
Catalysts for aromatic conversion processes generally comprise zeolitic molecular sieves. Examples include, zeolite beta (U.S. Pat. No. 4,891,458); zeolite Y, zeolite omega and zeolite beta (U.S. Pat. No. 5,030,786); X, Y, L, B, ZSM-5 and Omega crystal types (U.S. Pat. No. 4,185,040); X, Y, ultrastable Y, L, Omega, and mordenite zeolites (U.S. Pat. No. 4,774,377); and UZM-8 zeolites (U.S. Pat. No. 6,756,030 and U.S. Pat. No. 7,091,390). It is known in the art that the aromatic feed stream to aromatic conversion processes often contains nitrogen compounds, including weakly basic organic nitrogen compounds such as nitriles, that can, even at ppm and ppb levels, cumulatively act to poison the downstream aromatic conversion catalysts such as aromatic alkylation catalysts and significantly shorten their useful life. A variety of guard beds having clay, zeolite, or resin adsorbents to remove one or more types of nitrogen compounds from an aromatic hydrocarbon stream upstream of an aromatic conversion process are known in the art. Examples include: U.S. Pat. No. 7,205,448; U.S. Pat. No. 7,744,828; U.S. Pat. No. 6,297,417; U.S. Pat. No. 5,220,099; WO 00/35836; WO 01/07383; U.S. Pat. No. 4,846,962; U.S. Pat. No. 6,019,887; and U.S. Pat. No. 6,107,535.
It has recently been discovered that unsaturated aliphatic hydrocarbons such as olefinic compounds, and particularly diolefins, can shorten the effective life of adsorbents, e.g. nitrogen adsorptive zeolites or molecular sieves, used in nitrogen guard beds that are applied to various process streams, including aromatic hydrocarbon feeds upstream of an aromatic conversion process such as alkylation. These unsaturated aliphatic, e.g. olefinic, compounds are present in aromatic process streams contaminated with nitrogen compounds, including benzene streams generated in styrene process separation sections and other streams requiring removal of the nitrogen compounds prior to being contacted with a catalyst or other material susceptible to nitrogen poisoning. The presence, in particular, of highly unsaturated olefinic compounds, e.g. C4-C6 diolefins, in aromatic streams having nitrogen compound contaminants, adversely impacts the performance of nitrogen adsorptive materials. Without being bound by theory, it is believed that the olefinic compounds and/or other unsaturated aliphatic compounds may shorten the life of the nitrogen adsorbent by competing with the nitrogen compounds for the adsorption sites and/or reacting, e.g. with aromatics such as benzene, to form heavy reaction products that deposit on the nitrogen guard bed adsorbent.